A question about (Solving a tax rate or interest rate problem using a system of linear equations)

Question

Last year, Shen had $20,000

 to invest. He invested some of it in an account that paid 9%

 simple interest per year, and he invested the rest in an account that paid 7%

 simple interest per year. After one year, he received a total of $1780

 in interest. How much did he invest in each account?

Answer 1

.09(X) + .07(20,000-X) = 1780

.09X -.07X = 1780 - 1400 = 380

.02X = 380

X = 380/.02 = 19,000 invested at 9%

20,000-X = $1,000 invested at 7%

1780/20000 = .089 = 8.9% which is very close to 9%, so most all the investment is in 9%, not 7%

8,9 is within 0.1 of 9

while 8,9 is within 1.9 of 7

1.9 = 19 x 0.1, so 19,000 = 19 x 1,000